1. Field of the Invention
The present invention relates generally to electromagnetic valves, and in particular to an analog displacement type electromagnetic valve.
2. Description of the Prior Art
Analog displacement type electromagnetic valves find use in applications where the amount of controlled fluid is to be proportional to the amount of input current resulting from an input signal. However, it has been difficult to provide a valve capable of providing, at a reasonable cost, a desired degree of proportionality with a fast response characteristic to the input signal.
In a prior art analog displacement type electromagnetic valve as disclosed in Japanese Utility Model No. Sho 45-13723, a plunger having a tapered section near its end is longitudinally movably disposed within a hole of a stationary core. Bias is provided for the plunger in opposition to magnetic attraction generated by excitation of the core and the plunger with a coil mounted in a space therebetween. A first air gap is formed between the opposite end of the plunger and a first working surface formed at the center axis of the stationary core opposite to the core hole and a second air gap is formed between the tapered section of the plunger and the core hole. The attractive force that produces the longitudinal displacement of the plunger is inversely proportional to the square of the spacing of the first air gap. The spacing of the second air gap increases in proportion to the amount of plunger longitudinal displacement to add a corresponding amount of magnetic resistance to the magnetic circuit formed by the core and the plunger so as to compensate for the tendency of the longitudinal force to increase toward the first air gap. Although linear proportionality is provided, the magnetic flux in the second air gap generates a lateral force which causes the plunger to become transversely misaligned from the center axis of the core hole if the valve is not precisely machined. Misalignment of the plunger brings it into contact with the wall of the core hole, so friction prevents continuous movement of the plunger as a function of input current. Furthermore, the prior art valve requires a substantial amount of power because of the low operating efficiency for a given number of coil ampere-turns.